Geologists and geophysicists collect data regarding underground formations in order to predict the location of hydrocarbons such as oil and gas. Traditionally, such information is gathered during an exploration phase. In recent years, however, the art has advanced to allow the collection of geophysical and geological data as a well is being drilled.
For example, in Vertical Seismic Profiling (“VSP”), drilling operations are interrupted to place a series of seismic sensors at discrete depths in a borehole. A surface source releases energy that is reflected off underground geological formations. The seismic sensors in the borehole sense the reflected energy and provide signals representing reflections to the surface for analysis.
In a subsequent development, known as “drill bit seismic”, seismic sensors are positioned at the surface near the borehole to sense seismic energy imparted to the earth by the drill bit during drilling. This sensed energy is used in the traditional seismic way to detect reflections from underground geological formations. Further, this technique is used to detect “shadows”, or reduced seismic energy magnitude, caused by underground formations, such as gas reservoirs, between the drill bit and the surface sensors.
A greatly simplified description of those steps involved in drilling an oil well follows. A portion of the oil well is drilled using a drill string consisting of drill pipe, drill collars and drill bit. After a portion of the well has been drilled, a section of casing, or large bore pipe, is inserted into the well bore and cemented for, among other things, zonal isolation. Casing performs a number of functions, including: preventing the bore hole from caving in; preventing fluids in the bore hole from contaminating the surrounding formations; preventing the introduction of water into the surrounding formations; containing any production from the well; facilitating pressure control; providing an environment for the installation of production equipment; and providing zonal isolation.
When the casing is in place it is cemented to the formation wall. This is accomplished by pumping cement through the casing until it exits at the end of the casing through a special section of casing called a “casing shoe” and flows up the annulus between the casing and the wall of the well bore. The concrete is then allowed to set.
In subsequent drilling operations, the deep end of the newly cemented casing is drilled out and another section of the well bore is drilled. The process of drilling sections of the well bore followed by inserting and cementing well casing repeats until the desired well depth is reached.
As the well bore is being drilled, drilling fluids, known as “mud”, are pumped into the drill string. The mud travels down the drill string until it is ejected. The mud picks up cuttings and carries them to the surface. The specific gravity of the drill mud is carefully controlled so that the weight of the column of mud is (1) large enough to prevent gas or other hydrocarbons from entering the borehole from the surrounding formations; (2) without exerting so much pressure that the surrounding formations are damaged.
After each section of casing is laid and cemented in, the fracture pressure of the formation just below the end of the casing is measured. Generally, the fracture pressure of deeper formations is greater than the fracture pressure of shallower formations. The specific gravity of the drilling mud is subsequently controlled to make sure that the pressure on the formation at the end of the casing does not exceed the fracture pressure of the formation at that point. This is generally accomplished by calculations incorporating the measured specific gravity of the drilling mud and the depth of the column of drilling mud above the formation.
Downhole data are captured using “wireline” techniques in which, prior to casing being laid, a tool, such as an acoustic logging tool, is lowered into the well bore and slowly retrieved, gathering data and storing it or transmitting it to the surface as the tool is being retrieved. Alternatively, measurement while drilling (“MWD”) or logging while drilling (“LWD”) tools are attached to the drill string just above the drill bit and drill collars. These generally expensive tools gather data during the drilling process and store it or transmit it to the surface.